Probing structure in invisible protein states with anisotropic NMR chemical shifts

A general method for obtaining quantitative structural information on invisible, excited protein states by solution-based NMR spectroscopy is presented. The approach exploits relaxation dispersion techniques in which changes in chemical shift between ground and excited states are monitored in solutions with and without small amounts of residual molecular alignment. This allows the calculation of differences in chemical shifts induced by alignment that can be directly related to molecular structure, in cases where the orientation and magnitude of the chemical-shift tensor are well defined. An example using carbonyl chemical shifts as probes of a protein-ligand binding reaction is presented to illustrate and validate the method.